Leave the last of citrus jam (5)
I believe the answer is:
scram
'leave' is the definition.
(scramming is a kind of leaving)
'the last of citrus jam' is the wordplay.
'the last of' indicates one should take the final letters.
'jam' becomes 'cram' ('cram' can be a synonym of 'jam').
The final letter of 'citrus' is 's'.
's'+'cram'='SCRAM'
(Other definitions for scram that I've seen before include "Hop it!" , "Skedaddle!" , "In slang, go away and beat it" , "Buzz off!" , "Go away quickly" .)